The present invention relates to a semiconductor device including a nonvolatile memory, particularly a semiconductor device including a nonvolatile memory cell having a charge storage film formed of an insulating film capable of storing therein charges, e.g., a semiconductor device including a MONOS (Metal Oxide Nitride Oxide Semiconductor) nonvolatile memory cell, and a manufacturing method thereof.
Semiconductor memories can be roughly divided into a “volatile memory” which loses a memory content when a power supply is turned off and a “nonvolatile memory” which retains a memory content even when a power supply is turned off. Nonvolatile memories have been developed as storage media mainly for storing data and programs for devices such as a personal digital assistant, a digital camera, and a personal computer. Nonvolatile memories include products of which higher reliability is required than that required of a typical nonvolatile memory, which are for, e.g., in-vehicle use, medical equipment, and the like. The products use memory cells each having a stacked insulating film structure in which a silicon nitride film is interposed between silicon oxide films, and are known as MONOS (Metal Oxide Nitride Oxide Semiconductor) nonvolatile memories. In a method of storing charges in a floating gate, which is generally widely used for a nonvolatile memory, a defective bit resulting from a defect in a tunnel oxide film may occur. By contrast, the MONOS nonvolatile memory uses a discrete storage method which stores charges in a silicon nitride film, and is therefore characterized by having a high-reliability charge retention property without depending on the state of the tunnel oxide film.
In Patent Document 1 (Japanese Unexamined Patent Publication No. 2006-12382), a so-called split-gate MONOS nonvolatile semiconductor memory device is disclosed in which a memory gate for operating a memory and a selection gate for performing cell selection are separately formed.
In Patent Documents 2 and 3 (Japanese Unexamined Patent Publications Nos. 2002-289710 and 2009-105426), it is stated that, in a nonvolatile memory, silicon dots are discretely formed in a tunnel insulating film between a silicon substrate and a charge storage portion. Thus, a technology is disclosed which uses a Coulomb blockade effect to reduce the likelihood of charge loss, and improves the data retention property of a memory cell.